1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for reducing a Peak to Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
OFDM is widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Television, Wireless Local Area Networking (WLAN), Wireless Asynchronous Transfer Mode (WATM), etc. Although the OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme, the OFDM scheme has a characteristic that it can obtain the optimal transmission efficiency during high-speed data transmission by transmitting data while maintaining orthogonality between multiple subcarriers, and can also obtain the optimal transmission efficiency during high-speed data transmission as it has high frequency efficiency and is robust against multipath fading.
In addition, OFDM has high frequency efficiency and is robust against frequency-selective fading since frequency spectrums are used on an overlapping basis, and can reduce an Inter-Symbol Interference (ISI) effect by using a guard interval. Further, OFDM enables simple design for a hardware structure of an equalizer, and is robust against impulse noises. Due to such advantages, OFDM is being actively used for the communication systems.
However, despite its many advantages, the OFDM system causes a high Peak to Average Power Ratio (PAPR) due to multi-carrier modulation, i.e., since OFDM transmits data using multiple carriers, the final OFDM signal has a significant change in amplitude, as the signal's amplitude becomes a sum of amplitudes of the carriers. In addition, if phases of the carriers are coincident with each other, the amplitude has a very high value.
Such a high-PAPR signal, however, may get out of a linear operation range of a High Power Amplifier (HPA). Therefore, the high-PAPR signal may suffer from distortion after passing through the HPA, causing a reduction in the system performance.
To address the high-PAPR problems of the OFDM system, several PAPR reduction techniques have been provided, and include Clipping, Coding, SeLected Mapping (SLM), Partial Transmit Sequence (PTS), Tone Injection (TI), Tone Reservation (TR), etc.
Of the PAPR reduction techniques, the TR scheme reserves L tones in N subcarriers, and transmits no data on the L reserved tones to reduce the PAPR. In this case, a receiver disregards the L tones that have transmitted no information signal, and restores information signals only on the (N-L) tones, contributing to a decrease in complexity of the receiver structure.
A gradient algorithm is one of the typical methods of using the L reserved tones to reduce the PAPR in the TR scheme. The gradient algorithm is provided by applying a method similar to the Clipping technique to the TR scheme.
The gradient algorithm is used to generate signals having the impulse characteristics using L tones that transmit no information signal, and to clip output signals of Inverse Fast Fourier Transform (IFFT). If signals having the impulse characteristics are added to the IFFT's output signals, data distortion occurs only on the L tones, and no data distortion occurs in the other frequency regions.
In the Clipping technique, the noises generated by clipping may affect all subcarriers. However, in the TR technique, the noises generated by clipping affect only some reserved subcarriers rater than affecting all the subcarriers.
The gradient algorithm optimizes impulse waveforms so that the peak of IFFT output signals is reduced in the time domain. PAPR-reduced signals, which are generated by adding a sum of impulse waveforms optimized by the gradient algorithm to the IFFT's output signals, are transmitted to a receiver.
Then the receiver, since the receiver already has information on positions of L tones, only needs to receive data on the remaining subcarriers except for the L reserved tones.
With reference to the accompanying drawings, the TR method will now be described.
FIG. 1 is a diagram illustrating a structure of a general TR scheme-based transmitter.
An (N-L)-point input signal (X) 105 and an L-reserved tone signal 110 are input to a tone reservation unit 120, and the tone reservation unit 120 reserves the L-reserved tone signal 110 in a subcarrier position previously agreed upon between a transmitter and a receiver. In this case, zero (0) is inserted into the L tones, with no data transmitted thereon. When the parallel data X and a sum of the L reserved tones, output from the tone reservation unit 120, are input to an N-point IFFT unit 130, the N-point IFFT unit 130 performs an IFFT calculation on the input data, and outputs the result to a Parallel-to-Serial (P/S) converter 140. Then the P/S converter 140 generates a time-domain output signal X by processing the input signal. Next, a gradient algorithm unit 150 transmits a transmission signal X+C obtained by adding a signal C generated by the gradient algorithm unit 150 to the output signal X of the IFFT unit 130. In this case, the gradient algorithm unit 150 calculates the signal C so that a PAPR of the output signal X is reduced, using an impulse waveform read from a memory 160.
The signal C added to L tones to reduce the PAPR is determined as follows. L subcarriers are previously reserved and used for a code C; positions of the L subcarriers are fixed by the tone reservation unit 120 during their initial transmission and remain unchanged during data transmission. The code C can be expressed as Equation (1).
                              C          k                =                  {                                                                                          C                    k                                    ,                                                                              k                  ∈                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                                        0                  ,                                                                              k                  ∉                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                          (        1        )            
In Equation (1), k denotes an index of the tone reservation unit 120. In this case, an input signal X is reserved in a subcarrier other than the code C as shown in Equation (2), and i represents the position of the reserved tone in each FFT.
                              X          k                =                  {                                                                                          X                    k                                    ,                                                                              k                  ∉                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                                        0                  ,                                                                              k                  ∈                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                          (        2        )            
Minimization of PAPR is achieved by optimizing these L subcarriers. {tilde over (C)} is optimized by Equation (3) so that PAPR is low.
                              C          ~                =                  Arg          ⁢                                          ⁢                                    min                              C                ~                                      ⁢                          (                                                max                                      n                    =                                                                  0                        ⁢                                                                                                  ⁢                        …                        ⁢                                                                                                  ⁢                        N                                            -                      1                                                                      ⁢                                                                                              x                      n                                        +                                          c                      n                                                                                                    )                                                          (        3        )            
In Equation (3), cn denotes an nth element value of a time-domain vector obtained by performing IFFT on a vector C. To find an optimized signal of C, calculation of Equation (3) is performed. To solve Equation (3), complex linear calculation should be performed. In actual implementation, however, the gradient algorithm is used, which can obtain the similar performance only with simple calculation.
The code C is optimized so as to remove the peak value of a vector x. If xclip is defined as a vector where x is clipped to a certain level A, Equation (4) is derived therefrom.
                              x          -                      x            clip                          =                              ∑            i                    ⁢                                    β              i                        ⁢                          δ              ⁡                              [                                  n                  -                                      m                    i                                                  ]                                                                        (        4        )            
In Equation (4), βi denotes a clipping value, mi denotes a position where a corresponding vector is clipped, and δ denotes an impulse function.
If c is defined as in Equation (5) below, Equation (6) can be derived therefrom, making it possible to reduce the peak value of a transmission symbol.
                    c        =                  -                                    ∑              i                        ⁢                                          β                i                            ⁢                              δ                ⁡                                  [                                      n                    -                                          m                      i                                                        ]                                                                                        (        5        )                                          x          +          c                =                  x          clip                                    (        6        )            
Therefore, c can be construed as a sum of delayed and scaled impulse functions. However, in the frequency domain, Ĉ=FFT(c) has a non-zero value in most frequency positions, and distorts values of data symbols except for L reserved positions. Therefore, in the frequency domain, a waveform having a characteristic of an impulse function, which is affected only in the L reserved positions and is not affected in the other positions, must be used for clipping.
A waveform having the impulse characteristics is designed as follows.
For example, 1L is a vector having a value 1 in L reserved positions and a value 0 in the remaining positions, and p is defined as Equation (7).
                    p        =                              p            ⁡                          [              n              ]                                =                                    [                                                p                  0                                ⁢                                  p                  1                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  p                                      N                    -                    1                                                              ]                        =                                                            N                                L                            ⁢                              IDFT                ⁡                                  (                                      1                    L                                    )                                                                                        (        7        )            
In Equation (7), p0=1, p1 . . . pN-1 and each has a much smaller value compared with p0. When p[((n−mi))N] is defined as a value obtained by circular-shifting p by mi, even though the value undergoes Discrete Fourier Transform (DFT), only its phase varies and the value has a value 0 in the positions other than the L reserved positions in the frequency domain.
In designing the waveform having the impulse characteristics as stated above, the waveform cannot become similar to the ideal impulse waveform unless the waveform is designed such that a size of the remaining p1 . . . pN-1 except for p0 is small. As the size of p1 . . . pN-1 is smaller, a change in size of other signals in the positions except for the position of p0 is smaller during execution of clipping. If p1 . . . pN-1 are designed to be large, the peak of other signals may increase again in the clipping process, causing a decrease in the PAPR reduction performance.
Since the positions of the L reserved tones determine an impulse waveform and the impulse waveform exerts influence on the PAPR reduction performance as stated above, one well-designed reserved tone position is generally previously determined in applying a tone reservation method to the OFDM system. A time-domain impulse waveform generated by this reserved tone is also previously stored. Through this process, it is possible to avoid calculating positions of reserved tones and an impulse waveform for every symbol.
FIG. 2 illustrates a frame structure of a general broadcast communication system. Several OFDM symbols constitute one frame, and a structure of pilot tones shows a scattered structure where positions of the pilot tones change in every OFDM symbol. Since such pilot tones are used for channel estimation, the pilot tones should not undergo interference and/or distortion.
However, when the above-stated structure of predetermined reserved tones is used, a collision may occur between the reserved tones and the pilot tones in the frame structure of FIG. 2.
FIG. 3 illustrates a collision occurring between pilot tones and reserved tones when one reserved tone based on the conventional tone reservation scheme is used in the frame structure of FIG. 2.
In other words, while no collision occurs between the reserved tones and the pilot tones in the case 301 of FIG. 3, a collision may occur between the reserved tones and the pilot tones in the cases 303, 305, and 307. Therefore, there is a demand for an apparatus and method for designing and managing reserved tones while avoiding the collision between the pilot tones and the reserved tones.